The invention relates to a method for contact-free interval or thickness measurement in which a sharply concentrated light ray beam generated by a light source is periodically deflected by means of a beam deflector over a measuring space. A time difference between a standard and at least one measuring signal represents a measure of the interval or the thickness, and in which the time difference between a reference signal and the standard signal as well as between the standard signal and a first deflection signal is employed by means of comparison with two given fixed time differences for the control of the deflection frequency and the deflection amplitude of the light ray beam.
This method and the corresponding device for its implementation are known from the Siemens Forschungs- und Entwicklungsberichten, Volume 4, (1975), Number 6, pages 336-344 and Volume 6 (1977), Number 3, pages 180-188, incorporated herein by reference. The time difference between the reference signal and the standard signal holds the beam deflector at the resonant frequency. The time difference between standard and first deflection signals holds the deflection velocity constant. As long as the direction of the light ray beam does not change, the measuring time also remains stable given an unchanged interval.
It has been proven, however, that because of thermal influences of the beam deflector and on the light source, the direction of the light ray beam can change. Since the reference signal is generated independently of the direction of the light ray beam, given a change of direction, the time difference between a reference and a standard signal changes, since--depending on the displacement of the direction of the light ray beam by a positive or negative angle--the deflected light ray beam reaches the light-sensitive detector responsible for the standard signal in a shorter or longer time via reflection from a reference plane. This deviation of the measured time difference is compensated via a change of the deflection frequency. On the other hand, a changed deflection frequency changes the deflection time, i.e., the time difference between the standard and a first deflection signal. The time difference between standard and first deflection signal is again stabilized via a change of the deflection amplitude.
The changed deflection amplitude, however, changes the measuring time and thus causes a measuring error, above all in the upper measuring range.
To hold the direction of the light ray beam constant with opto-mechanical techniques in the beam deflector and in the light source would require an unjustifiable expense.